1. Field of the Invention
The present invention generally relates to an image compression/decompression system, in particular, to an image compression/decompression system employing efficient pixel thinning-out and interpolation schemes.
An image compression/decompression system according to the present invention may be applied widely to various image processing application programs, various device drivers (software) such as printer drivers, also, to various image processing apparatus/equipment processing color images.
2. Description of the Related Art
With regard to image compression/decompression techniques, various schemes have been proposed. For example, Japanese laid-open patent application No. 7-221993 discloses a color image data thinning-out method, and a color image data compression method. In the methods, according to change rates in U data and V data representing color components, a size of pixel zone to be thinned out for each image block is controlled. Specifically, as the change rate is larger, the size of pixel zone to be thinned out is to be reduced. On the other hand, as the change rate is smaller, the size of pixel zone to be thinned out is to be enlarged. Thereby, an edge line of an image is prevented from being lost due to thinning-out processing.
Thus, by utilizing a characteristic of human eyes, a thinning-out rate is appropriately selected for a color component according to a color change rate for each image block such as that including 8×8 pixels, 16×16 pixels or the like. Thereby, effective image compression without remarkable apparent degradation in image quality may be attained.
However, according to the above-described methods disclosed by Japanese laid-open patent application No. 7-221993, a considerable time may be required for calculating change rates in U data and V data for each image block.
Furthermore, generally speaking, a natural image is likely to include a lot of thin lines/edge lines extending horizontally or vertically while a few of those extending obliquely. Therefore, when simply pixels arranged vertically are thinned out as disclosed by the above-mentioned patent publication as shown in FIG. 1, a lot of thin lines, edge lines and so forth included in a natural image may be completely lost during image data compressing processing.